1. Field of the Invention
The present invention relates to materials metering devices, and more particular to metering devices for the handling of granulate or pulverulent or highly viscous plastic raw materials which are being fed to plastic materials processing machines, such as injection molding machines and extruders, for example.
2. Description of the Prior Art
A wide variety of plastic materials metering devices are known from the prior art. This invention concerns itself specifically with the type of metering device in which a supply of raw material is stored inside an upright, generally cylindrical receptacle whose bottom section accommodates therein a rotating metering disc. The metering operation itself is performed by a number of apertures or metering pockets arranged at or near the periphery of the metering disc which, as it rotates, receives fixed amounts of raw material in its metering pockets and discharges the raw material through a discharge opening in the bottom section of the receptacle in a regular succession of uniform charges of raw material. While the metering pockets are open on the upper and lower sides of the metering disc, their lower side is normally covered and closed by the bottom wall of the receptacle bottom section, and their upper side is covered and closed off against the raw material in the receptacle in at least that angular range of travel in which the discharge through the receptacle bottom wall takes place. The metering pockets on the metering disc may be either a series of vertical throughbores arranged in the vicinity of the disc periphery, or they may be in the form of appropriate radial recesses on the disc periphery itself. These recesses require that a vertical wall portion of the receptacle bottom section engages the rotating disc with minimal clearance, in order to constitute an outer closure for the metering pockets.
A plastic materials metering device featuring the first-mentioned type of rotating metering disc is disclosed in German Pat. No. 1,947,405, where the metering disc has metering pockets in the form of throughbores arranged just inside the periphery of the metering disc. The raw material receptacle is a cylindrical drum which is supported on a matching bottom section, the circular metering disc having the same outer diameter as this drum, being positioned just underneath the latter, between it and the bottom section of the receptacle. In the angular region of the discharge opening in the receptacle bottom section is arranged a horizontally extending cover plate which prevents raw material from entering that metering pocket whose contents are just being discharged from the metering device. Ahead of the discharge position on the periphery of the receptacle is further arranged a stationary guide plate having a guide surface extending over the metering pockets in a slightly inclined orientation so as to exert a tamping and wiping effect on the contents of the metering pockets.
While the raw material carried inside the metering pockets enters the latter through gravity and is thereafter slightly compacted in a tamping action, the discharge of the material from the metering pockets is left entirely to gravity. This type of discharge may, however, amount to a serious shortcoming in connection with raw materials which have a pronounced tendency to pack together. These materials, especially the pulverulent type of materials, will adhere to the sides of the metering pockets and return to the receptacle, instead of falling out of them, when the pockets are opened. When this happens, the amounts of raw material which are actually being discharged are smaller than the intended amounts. The result is an uneven rate of material discharge and an overall deficit of material supplied to the mixing funnel of the plastic materials processing machine.
In the U.S. Pat. No. 2,366,379 is described a disctype metering device, where an attempt has been made to solve this problem by forcibly removing residual material from the metering pockets. A spring-loaded plunger is arranged above the metering pockets in the discharge position which, when a filled pocket arrives, enters into the latter and expulses its contents downwardly.
This arrangement, in order to work satisfactorily, requires a close fit between the plunger and the pockets and the latter must remain immobile while the plunger is engaged. The disc drive must therefore be intermittent and the plunger needs a separate drive means. Such an apparatus is slow and expensive.